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ELECTROMAGNETIC WAVES

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They have a large amount of energy Energy carried by electromagnetic radiation Energy frequency We calculate the energy of the radiation using: E ... – PowerPoint PPT presentation

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Title: ELECTROMAGNETIC WAVES


1
ELECTROMAGNETIC WAVES
  • SECONDARY 3 PHYSICS

2
WHAT ARE EM WAVES?
  • Electromagnetic waves (EM waves for short) are
    waves that can travel in a vacuum.
  • These waves are created by the vibration of an
    electric charge.

3
  • EM radiation is a wave that is produced as
    follows
  • When charges accelerate, they produce a changing
    magnetic field.
  • This changing magnetic field creates a changing
    electric field at 90 to it.
  • The electric field now causes a changing magnetic
    field at right angles to it.
  • The magnetic and electric fields are able to
    generate each other without any decrease in
    strength if the fields move at 3 ? 108 m.s-1.

4
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5
Electromagnetic Spectrum
  • It is a group of different electromagnetic waves.
  • There are 7 components in the spectrum.

6
Electromagnetic Spectrum
  • Gamma rays
  • X rays
  • Ultraviolet
  • Visible light
  • Infra-red
  • Microwaves
  • Radio waves

Highest frequency Shortest wavelength
Lowest frequency Longest wavelength
7
Uses of Gamma Rays
  • Kill cancer cells
  • Study the nucleus in atoms

8
Uses of X-rays
  • Take pictures of bones in the body
  • Study the crystal structure of crystalline
    substances
  • Check for cracks in metal plates

9
Uses of Ultraviolet
  • Detect counterfeit notes
  • Gives the clothes a glow effect in discotheques
  • Sun-tanning
  • Sterilise medical equipment

10
Uses of Visible Light
  • Enable us to see things
  • Photosynthesis in plants

11
Uses of Infra-red
  • Heating
  • Haze photography

12
Uses of Microwaves
  • Radar communication
  • Analysis of the molecular and atomic structure
  • Telephone communications

13
Uses of Radio Waves
  • Radar communications
  • TV and radio broadcasting

14
Common Properties of EM Waves
  • All transverse waves
  • All travel at the speed of light. (3 x 108 m/s)
  • Can travel through solid, liquid, gas and vacuum
  • Obey the laws of reflection and refraction
  • All can be absorbed and emitted by matter
  • The wave equation is applicable to all

15
Penetrating ability of electromagnetic radiation
  • The ability of EM radiation to go through
    (penetrate) bone, glass or concrete depends on
    the energy of the radiation and then also on the
    frequency of the radiation.
  • Radio waves have the lowest frequency of EM
    radiation and thereby the lowest energy. They are
    not able to travel through the ground, therefore
    we cannot receive a radio signal underground. The
    radio waves are strongly diffracted (bent) around
    objects, while the shorter radio waves are
    reflected by the charged upper atmosphere
    fluctuate because of the changing reflecting
    ability of this layer.
  •  

16
  • Microwaves have a low penetrative ability. Cell
    phone masts that use microwaves must have no
    obstructions in between them.
  • Infrared radiation has low energy. It is able to
    warm the skin but cannot penetrate through the
    skin.
  • Ultraviolet radiation has a frequency higher than
    violet light in the visible spectrum. The energy
    is such that it is harmful to the eyes and can
    cause the skin to tan.

17
  • X-rays have a high energy and a high penetrative
    ability. They can penetrate the soft tissue in
    the body but cannot go through the bone.
  • Gamma rays can pass through a few centimetres of
    lead or concrete. They have a large amount of
    energy

18
Energy carried by electromagnetic radiation
  • Energy ? frequency
  •  We calculate the energy of the radiation using
  •  E h ? f
  •  Where E is the energy of the radiation measured
    in Joules (J)
  • h is plancks constant and has a value of
  • 6,63 ? 10-34 J?s
  • f is the frequency of the radiation measured in
    hertz (Hz)
  •  

19
  • Since f c/ ?
  •  To calculate the energy of the photon, we use
    the following equation
  • E hc/ ?
  • Where E Energy in joules (J)
  • h planks constant (6,6 ? 10-34 J.s)
  • f frequency in hertz (Hz)
  • ? wavelength in metres (m)
  • c speed of light (3 ? 108 m.s-1)
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